Replacement of video data contained in MPEG (Moving Picture Experts Group) program streams is well established. Normally MPEG programs are encapsulated within a MPEG Transport stream supporting many channels. A primary requirement is that data bandwidth of the Transport stream must be held below the capabilities of the transmission system. Thus when video data replacement occurs the defined bandwidth of the transmission system cannot be exceeded otherwise loss of all programs may occur at the receiving end due to lost data packets.
There are many situations where video replacement is required. For example news breaks may occur for local news that replace a national broadcast. Similarly local advertising may replace national advertisements in advertising breaks. Local programming is stored ready for play out at the appropriate time. However, in such cases, bit rates of the original program and replacement program can vary, meaning peaks of data occur differently because the bandwidth of video changes according to motion in the picture. Data packets in Transport streams are organized such that bandwidth utilization is often close to maximum, therefore additional variations by program replacement can result in excess bandwidth.
Video data replacement is often referred to as “splicing,” as opposed to physical switching, since data is being replaced. In commercial products, ‘Transport stream splicers’ perform program replacement. This is usually achieved by de-multiplexing and re-multiplexing data packets in order to keep within transmission bandwidth limits. A presentation time stamp (PTS) and/or a program clock reference (PCR) may be altered for program streams.
It is much more difficult to splice variable bit rate programs as transport stream splicers perform a statistical analysis of on-going bandwidth requirements and adjust placement of data packets accordingly. Variable bit rate program replacement can result in transmission bandwidths being exceeded. In these instances it may be necessary to perform decoding of the video data to uncompressed form and re-encode the uncompressed video data dynamically at lower bit rates for the period that transmission bandwidth is exceeded. In this case, lower quality of video is created for a few frames, but provided the period is short, the lower quality video can be difficult to detect.
There are products that perform program replacement without de-multiplexing or re-multiplexing the transport stream data. However, there are disadvantages in these products in that these products rely on the use of constant bit rate program streams, and the program streams being spliced need to have similar bit rates if transmission bandwidth is close to the limits, which is typically the operating environment.